Interferometric synthetic aperture radar (IFSAR, also abbreviated as InSAR) is a specialized radar technique for using phase interferometer methods between two spatially displaced high resolution (complex) SAR images to generate high quality terrain elevation maps called digital elevation maps (DEMs). The high spatial resolution of IFSAR imagery enables independent measurements of terrain elevation on a dense grid of sample points on the order of several decimeters to several 10s of meters. Furthermore, the use of active microwave radar as the sensor of choice inherently provides a (nearly) all-weather, day/night capability to generate orthorectified magnitude imagery and DEMs.
SAR systems use antennas that are designed to transmit and receive electromagnetic waves over a particular band and of specific polarization(s). It is well known that interactions between the incident transmitted wave and illuminated ground objects, the backscattered (reflected) wave can change the polarization of the scattered wave to be different from the polarization of the incident wave. Therefore, the radar antennas are often designed to simultaneously receive the different polarization components of the EM wave. For example, SAR antennas are frequently designed to generate electromagnetic waves with both horizontal (H) and vertical (V) linear polarizations. While only one polarization is transmitted from a single antenna at any given time both the H and V parts of the antenna(s) can receive the H and V components of the backscattered signal (the system electronics keep these signals separate). An exemplary collection platform for permitting single-pass full polarimetric SAR data is disclosed in the article entitled “Updating GeoSAR for full-Interferometric Capability,” IEEE Radarcon09 978-1-4244-2871, Jul. 9, 2012, which is incorporated herein in its entirety for its teaching of placement and types of sensors installed on an aircraft for such collection.
Single-pass, full-polarimetric (or “full-pol”) interferometry requires transmission and reception from two antennas in two polarizations. Transmission/reception diagrams are used to designate how the polarimetric data is acquired. FIG. 1 diagrams the possibilities. The circle (node) corresponds to an antenna, e.g., H1 corresponds to horizontal polarization for antenna-1, V2 corresponds to vertical polarization for antenna-2. The arcs connecting the nodes correspond to backscattered transmitted energy, the tail being the transmitting antenna, and the head being the receiving antenna. In general, the backscattered energy from each transmitted pulse can be received by each of the four antennas, therefore if all antennas are used (sequentially) for transmission and all antennas are used (simultaneously) for reception, then a maximum of 16 raw data channels are available for recording and image formation.
If the same-side antenna is used for both transmit (TX) and receive (RX), then the antenna index is dropped, since its physical location is specified. For example, for signals emanating from/to the same antenna, these shortened identifiers are frequently used, where the antenna location is understood:                HH—for horizontal transmit and horizontal receive,        VV—for vertical transmit and vertical receive,        HV—for horizontal transmit and vertical receive, and        VH—for vertical transmit and horizontal receive.        
When transmit and receive polarizations are the same, this is referred to as co-polarized (e.g., HH, VV); and, similarly, when the transmit and receive polarizations are the opposite this is referred to as cross-polarized (e.g., HV, VH). Cross-polarized signals are orthogonal to one another. The above discussion is based on linear polarizations, but the same principle applies if the two polarizations are right and left circularly polarized. If all the returns are acquired, circularly polarized signals mathematically can be converted to linearly polarized equivalents, and vice versa.
The ability to characterize sea ice is becoming increasingly important. The makeup of ice that covers various surfaces of the globe has been continually changing. However, the changing ice coverage and the characteristics of that ice have changed dramatically in recent years such that navigation of Polar Regions can now be accomplished. Further development of resources in these regions is also receiving additional attention.
Both environmental concerns and natural resources concerns have a great deal of interest in measuring and characterizing those regions of the earth that have some degree of ice coverage. For example, entities that are involved in the drilling and production of oil and gas in marine environments, especially those in Polar Regions, have a particular interest. In particular, when drilling and producing oil in arctic conditions, sea ice can pose a threat to both structures and personnel. Thus, the structure, thickness and location of this sea ice are critical components of the safety management plan.
Attempts using SAR and IFSAR have been made at measuring and characterizing sea ice. These have occurred using mostly satellite-based X-band systems, and to a much more limited extent, airborne systems. Further, there have been some examples of using interferometric SAR to map the top of ice through a few decimeters of dry snow. These systems can only characterize new from first-year and multiyear ice. However, such systems do not measure ice thickness.
Currently, ice thickness measurements are typically acquired by collecting profiles using low flying profiling radar systems or deploying on-ice tools, such as sled-based ground penetrating radar and drilling core samples. These systems, while very accurate, provide data coverage over very small areas.